I have a model which I need to separate its head from body
I'll try showing you some simple way to simplify (by hand) your model body: I downloaded a full body human mesh from this site. 

The archive is compressed with .rar format, so you have to unrar it before (it's like a .zip archive, just different): find a suitable program for your computer system (eg: http://www.7-zip.org/, free and open source).

The extracted model itself, is in .obj format so you can't open it in blender directly, you have to import it: blender has a "Wavefront (.obj)" importer by default, use it with default settings, it works well.

You get this model:



how to simplify it body without touching its head?

example: select a "loop" of vertices (this is a particular one that goes over all the model'storso)



how to?

select two vertices sharing an edge, like in the picture below, 

and in edit mode use the 

mesh > edges > edge loop

you can also get the same menu floating, like in the picture,  using the shortcut CTRLE



anyway you can select loops in any way, with the many selection tools of blender...


  tip: a useful simplified way to select a loop is to select one of its  edges while keeping pressed the ALT key (and, as always, SHIFT if you are adding to a previous selection)


now the loop is selected, 



now press X to get the "delete" menu, and use "dissolve edges"



and you get 



you can also "dissolve" several edge loops together:just leave some loop between them, like here:



and you'll simplify it faster



that's it, you reduced you vertex count without ruining your shape!

you can do this on arms, legs, torso, just be sure to do it simmetrically, and just where it don't alter too much your body shape and... don't loose your head :))

Hope it helps.

edit:

after importing your real model into blender (using mmd_tools addon), I saw it's quite dense, but made of triangles instead of quads... 



as said below in comments, loops work only on quads, but fortunately blender can convert triangles (tris) to quadratic shapes (quad) - 2 tris become 1 quad. with this menu command:



after which you get your model almost entirely in quads



Then I noticed another problem: there are doubles (how I noticed? some loop was still not working, so I guessed that... experience)

So you have to remove doubles first: in edit mode, press W to get the "specials" menu and press "remove doubles"



you see there were some



Now loop tools work as expected, and you can try my suggested path above



You may have noticed that converting 2 tris to 1 quad has reduced total faces count (of course) from about 45k to 25k. But exporting back your "quad" model to pmx, triangles will be restored by the exporting process (so 1 quad will become 2 tris). So, based on your question info, you'll have to reduce your "quad" model below 10k quads, acting on body obnly, I guess. It's a lot!

I also noticed your model is fully rigged, so it could be ready to be animated in VRchat? ...if yes, of course, reducing body vertices could make it move differently than intended, who knows. 

It's not as easy as those people that advised you to use blender thought perhaps. It can be done, imho, but it will require a lot of tries and patience, and restarts if it becomes a mess...

Last, about this comment:
"Is there a possibility just to make the head in a different part of the body's part so that I can decimate only the body using Unity? Because I have nice Unity plugin for reducing the Tris but need to make the head a different part or mesh than the whole body. "

you can split your mesh easily in blender, too. select head vertices, the press p, and select "selection", Done. And you can act on body only. But if you later need to join that head with the simplified body, it will be worse. And if the model is to be animated in VR chat, using the full armature that the model has, it will be even worse, trust me...

But you can try different things, at least!

edit2:

about your last comments, and the fact  that you can't reduce much triangles... yes, it is impossible to get below 20k triangles if you don't want to edit the head...



as you see, your head alone (selected) is made of over 12.2k quads, ie: around 25k triangles... that does not deend on blender tools, it's just how this bee is done.

If you want to go below 20k for the entire model, you must edit the head also... sorry...